The present invention relates generally to electronic ballasts for gas discharge lamps. More particularly, the present invention pertains to electronic ballasts capable of operating one or more lamps in both a full illumination (brightness) mode and a dimmed illumination mode—an illumination level less than the full illumination mode. Even more particularly, the present invention relates to electronic ballasts that can operate the lamps in a dimmed illumination mode and provide appropriate lamp filament heating to ensure stable and reliable operation of the lamp in the dimmed mode.
A crucial aspect of operating a gas discharge lamp, whether in a full illumination or dimmed illumination mode, concerns proper heating of the lamp's filaments. Without proper filament heating, the performance and reliability of the lamp(s) will degrade. Specifically, without proper heating, the filaments will not be able to thermionically emit electrons in sufficient quantities to maintain an arc (required for proper lamp operation). Further, attempts to operate the lamp while the filaments are not in the desired temperature range will result in damage to the filaments and/or the thermionically emissive material coating the filaments and lead to premature lamp failure.
The luminosity or brightness of the lamp and the level of filament heating are controlled, in large part, by the current passing through the filaments. For example, when the lamps are operated in a full illumination mode, the current passing through the filaments is at or near the lamp's maximum operational limits (to generate a light output adequate for the full illumination mode) and is typically sufficient to heat the filaments without additional assistance.
Conversely, when the lamp(s) are operated in a dimmed illumination mode, the current passing through the filaments is reduced. Although the reduced current presented to the filaments permits dimmed mode illumination, the reduced current will often not be adequate to reliably operate the lamp-depending on the extent of dimming. As such, supplemental heating must be supplied to the filaments to avoid the above-mentioned complications associated with deficient filament heating.
What is needed, then, is an electronic ballast that can operate a lamp in both full and dimmed illumination modes and provide supplemental heating to the filaments when the lamp is operated in the dimmed illumination mode in a reliable, cost-effective manner.